The invention relates to a solar cell.
Background of the Invention
U.S. Pat. No. 4,596,237 discloses a solar cell for obtaining transportable thermal energy. The cell has swivelling, slatted shading elements on the outside and/or a coating applied to the outer transparent cover, which limit the maximum incident thermal radiation. The energy taken up is transferred via subsequent absorber layers to a plurality of encapsulated latent heat stores which store the heat chemically and release it again to the adjacent rooms with a time delay on cooling, by resulting recrystallizations.
An air cushion forms between the heat stores, which are arranged opposite one another and are tightly sealed on all sides; only a very limited heat transfer can take place, so that the losses resulting from cooling are very high, which means that only a low heat flow to the inside of the building takes place with a relatively short time delay. A perceptible day/night compensation is therefore not possible due to the system. The solar converter containing chemicals is furthermore very expensive to produce and, depending on the storage medium used, is even an envimomental hazard and requires special disposal measures on later replacement or breakage.
A low-energy house with a honeycomb structure which absorbs radiant heat is known (Felix Schmid, Wxc3xa4rmedxc3xa4mmung mit Karton, Schweizer Energie-Fachbuch [Heat Insulation with Cardboard, Swiss Energy Textbook] 1988, Verlag Kxc3xcnzier-Bachmann AG, CH-9001 St. Gallen, p. 50-51). This structure comprises solar cells which are employed with a front glazing in elemental construction to build up a wall 30 to 40 cm thick. A wooden frame contains a conventional heat insulation and a cardboard honeycomb 8 cm thick positioned in front, which are protected by the rear-ventilated low-reflection glazing. The cardboard honeycomb acting as a solar collector reduces the temperature gradient in the outer skin of the house and minimizes the heat flow from the inside outwards. However, the actual energy gain is relatively small even on sunny days, since the solar radiation into the horizontal honeycombs made of recycled paper covers only the lower front region thereof and passes into the inside only by means of low thermal convection. The thermal time constant of the material used is inadequate per se and is intensified in its negative effect by the structure-related poor longitudinal conductivity of the honeycombs, which furthermore have channels with an approximately circular cross-section and a constant width.
In practical operation it has been found that on sunny days temperatures of up to more than 90xc2x0 C. partly arise on the cardboard honeycomb, while on a cloudy winter day xe2x88x923xc2x0 C. was measured at the same point. These high temperature gradients result in an accelerated ageing of the honeycomb material, the swelling properties thereof inter alia undergoing reversible deterioration.
AT-B-375 125 discloses an insulating faxc3xa7ade which has a stationary structure for shading, but its use is limited to south-side faxc3xa7ades. By horizontal channels closed on both sides in the insulating material, a direct introduction of thermal radition into the building is said to take place in the winter months. However, no storage of heat and also no time-delayed introduction of heat into the inside of the building is possible by this means. The necessary high number of channels causes high insulation losses, which has a particularly adverse effect if the temperature difference between the inside of the building and outside is high, i.e. in the absence of solar radiation and in particular in the late hours of the night.
Brief Summary of the Invention
The object of the present invention is therefore to provide a solar cell which does not have the abovementioned disadvantages and nevertheless has good summer/winter properties. The solar cell should achieve a temperature course in the house which is as balanced as possible, regardless of the height of the sun; allow a so-called comfort climate there, and in winter effect a noticeable solar heat gain. The solar cells must be aesthetically pleasing and it must be possible to integrate them both into new buildings and into existing structures, at least in a visually acceptable manner. The solar cell must be adaptable to the local circumstances of vapour diffusion (formation of dew). The structure thereof should also allow integration and/or combination with self-supporting structural elements known per se.
The materials used for the construction must moreover be biologically acceptable in building terms and as close to natural as possible; they should largely comply with the conditions of lasting construction.(according to the Brundtland Commission) and nevertheless be economical and maintenance-free. In particular, no expensive latent heat stores based on recrystallizations should be used in order to achieve the desired thermal time constant.
In accordance with the foregoing, a solar cell of the present invention includes a solar collector with a transparent, rear ventilated, cover. A stationery honeycomb structure for solar radiant heat absorption is at a geometry selected for the incident angle of the radiation, either with or without a front structure to further compensate for seasonal angle variations.
The term xe2x80x9cselective for the incident anglexe2x80x9d as used herein relates here to geometries and structures in and on honeycombs which, compared with those with only channels which are of uniform shape or slightly widened on the solar side, result in a better limiting of the summertime heating up temperatures and considerably reduce the heating requirement in winter.
In practice this means that only direct thermal radiation at an angle of  greater than 60xc2x0 to the perpendicular enters into the collector.
An embodiment of the solar cell which is open to vapour diffusion is important, which on the one hand serves for good transportation of heat in the day and night phases, and on the other hand prevents the formation of dew on separating layers. This embodiment can be implemented on the building object according to the sky direction and to geographical and local incident radiation circumstances.
The thermal time constant, calculated by conventional methods or determined and/or verified by experiments, is likewise adapted to the local circumstances and must guarantee a heat gain in the rooms lying behind during the night hours and delayed heating thereof during the day. In practice this leads to a reduction in the temperature gradient between the inside and outside on the building jacket.
When designing the insulation, because of obligatory building and energy regulations it is of course also necessary to reach compromises, which in physical terms often would not be necessary per se, since the invention relates to an active, self-regulating thermal system.
A volume ratio between the hollow spaces and closed volume of the honeycomb of  less than 4:1 regardless of the density of the material, results in a higher thermal capacity compared with the known cardboard honeycombs and therefore better storage properties.
The integration of mirrors to compensate for seasonal incident angle variation allows improved directing of the incident solar radiation. In the simplest case flat mirrors are used, these being assigned to the honeycomb openings, for example, as metal strips. A specific orientation of the mirrors adjusted to requirements in respect of the alignment of the wall surface or of the roof and the geographical position is particularly advantageous. The mirrors can also be of a bifocal construction.
By constructing the honeycomb with hollow spaces having larger dimensions in the direction of sun height as compared to an orthogonal direction, a higher introduction of energy can be achieved, so that rooms lying behind can be employed for drying numerous materials, or this type of solar cell can also be used in combination with heat exchangers for heating water or assisting heating. Such an arrangement may be appropriate in the roof area, where the highest energy yield is to be expected, corresponding to the inclination, which can be freely chosen within wide limits. Preheating of air in mechanical domestic ventilation systems and air-conditioning units can also be realized by means of simple cross-current heat exchangers.
Shading elements can be employed, and are particularly efficient in summer months and, with a size which is easy to optimize, do not interfere or interfere only slightly with the incident radiation in winter months.
By targeted structuring of the hollow spaces in the honeycomb, the heat absorption can be improved. By tapering or narrowing the hollow spaces in the inward direction the solar yield can be further increased or optimized. Inclined hollow results in an improved introduction of heat especially if the hollow spaces are not completely air-tight on the side of the heat insulation.
A transparent film may be employed for suppressing a backflow of heat caused by transportation of air in the region of the rear ventilation. The radiation yield is particularly efficient with use of a black radiation emitter film. The incorporation of further heat insulations in the honeycomb openings represents an expedient alternative to the films.
By the additional incorporation of known latent heat stores which release again during the night the heat excess present during the day, and on the other hand by so-called xe2x80x9cheat tubesxe2x80x9d, which transport the heat of evaporation from a first to a second zone and have the effect of a high heat insulation from the second to the first zone, the solar cell according to the invention may be further optimized.
Due to the high mass and the storage capacity which can thereby be chosen, a fine ceramic is particularly advantageous for the honeycomb, especially since this can be partly metal-coated economically favourably by glazes. Wood with a high hygroscopicity, results in a construction which is particularly close to natural and requires only little so called xe2x80x9cgreyxe2x80x9d energy.
The utilization of a transparent cover having a structured outer surface known per se which, without noticeable losses in energy transportation, generates iridescent light reflexes, depending on the height of the sun, can also be efficiently incorporated into the invention.